The field of the disclosure relates generally to gas turbine engines and, more particularly, to load reduction assemblies for use in gas turbine engines.
Gas turbine engines typically include a rotor assembly, a compressor, and a turbine. The rotor assembly includes a fan that includes an array of fan blades extending radially outward from a fan hub coupled to a rotor shaft. The rotor shaft transfers power and rotary motion from the turbine to the compressor and the fan and is supported axially with a plurality of bearing assemblies spaced axially along the rotor shaft. Additionally, the rotor assembly has an axis of rotation that passes through a rotor assembly center of gravity. Known bearing assemblies include rolling elements and a paired race, wherein the rolling elements are supported within the paired race. The rotor assembly is typically supported on three bearing assemblies, one of which is a thrust bearing assembly and two of which are roller bearing assemblies. The thrust bearing assembly supports the rotor shaft and supports axial and radial movement of the rotor shaft assembly. The remaining roller bearing assemblies support radial movement of the rotor shaft.
During operation of the engine, a fragment of a fan blade may become separated from the remainder of the blades and the rotor assembly. This is typically known as a fan blade out or a blade-off (FBO) event. Accordingly, a substantial rotary unbalanced load may be induced within the rotor assembly that is carried substantially by the fan shaft bearings, the fan bearing supports, and the fan support frames.
To reduce the effects of imbalanced loads, at least some known engines include support components for the fan rotor support system that are sized to provide additional strength for the fan support system. However, increasing the strength of the support components also increases an overall weight of the engine and decreases an overall efficiency of the engine when the engine is operated without substantial rotor imbalances.
Other known engines include a bearing support that includes a mechanically weakened section, or primary fuse, that decouples the fan rotor from the fan support system. During such events, the fan shaft seeks a new center of rotation that approximates that of its unbalanced center of gravity. This fuse section, in combination with a rotor clearance allowance, is referred to as a load reduction device (LRD). The LRD reduces the rotating dynamic loads to the fan support system. After the primary fuse fails, the pitching fan rotor often induces a large moment to a next closest bearing. The next closest bearing is known as the number two bearing position. The moment induced to the number two bearing induces high bending and stress loads to the fan rotor locally. To relieve the high bending stresses, the radial and pitching rotation stiffness of the number two bearing position are often softened or released during the FBO.
After FBO, the fan is typically allowed to rotate, in a process called windmilling, such that drag induced by the engine is reduced. However, during windmilling the loads induced by the rotor assembly and carried by the fan bearings are lower than during the FBO. As such, the LRD designed for FBO increases vibration within the engine during windmilling because stiffness of the number two bearing position is released.